1. Field of Invention
The invention relates to a device for the production of film tubes as defined in the generic clause of claim 1 and a method as defined in the generic clause of claim 8.
2. Description of the Prior Art
Film tubes are usually produced by means of blow-extrusion devices and methods. For this purpose, a die head is used, to which one or more melt streams made of plasticized plastics are supplied. Each melt stream is distributed in the form of a ring as uniformly as possible in the die head. If several melt streams are supplied, the individual melt layers are combined together. The thus molded single-layer or multilayered tube leaves the die head through an annular nozzle in a transport direction.
In order to cool down the film tube as rapidly as possible and bring it to a stable form, at least one fluid-application device, which surrounds the film tube annularly at least in certain regions, follows the die head in the transport direction. Air is frequently provided as the fluid, but other fluids, more particularly gaseous ones, are also possible. The fluid can be cooled or heated in sectors in order to be able to locally influence the thickness of the film tube. Its flow rate can also be modified.
The fluid is likewise conveyed by the fluid-application device and also by the film tube in the transport direction. After the fluid leaves the fluid-application device, a pressure equalization with the ambient air takes place, so that the cooling effect reduces rapidly.
The fluid-application device usually has a very limited overall height since the film tube is expanded in the radial direction by means of an internal pressure that is slightly higher than the ambient pressure. This determines the size of the film tube.
In order to increase the cooling effect of the fluid, the document CA 2 155 135 C suggests the provision of a ring that concentrically surrounds the annular region of the fluid-application device and projects above the same in the transport direction. Specifically, this means that the ring, which can be made of Plexiglas, is attached to the top end of the annular region. The ring can be followed by an additional ring in each case. Each additional ring has a larger diameter as compared to the preceding ring and is kept at a distance from the latter in the radial direction by means of spacers and is attached to the top edge of the preceding ring by means of screws. As a result of these rings, the cooling air remains longer on the film tube and the cooling effect is thus improved. Open regions are provided between the annular region and the ring or between two rings, and a pressure equalization can further take place through these open regions. Usually, additional air is drawn in through these open regions (known as the chimney effect) that can unfold an additional cooling effect. Since more heat per unit of time can now be released from the film tube, the production rate of the film tube is increased.
However, the solution suggested in CA 2 155 135 suffers from the shortcoming that the rings can be adjusted only by dismounting and mounting other rings when the tube size is changed. Furthermore, the annular nozzle of the die head is poorly accessible, which is a disadvantage particularly at the start of production when the melt oozing out of the annular nozzle must be pulled off manually.